Microwave transmitters for telecommunications satellites use an output multiplexer (OMUX) for combining the various transmission channels. In modern systems, it can be necessary to combine as many as 18 or more channels, and since the power of each channel in the Ku band (12 gigahertz (GHz) to 18 GHz) generally lies in the range 150 watts (W) to 250 W, the output multiplexer must be capable of accommodating total power levels of several kilowatts. In general, such a multiplexer uses a common manifold structure for combining the various channels. At the common output from the manifold, non-linear effects, e.g. due to connection flanges, lead to the appearance of interference signals due to intermodulation and known as parasitic intermodulation products (PIMP) which can occur in the passband of the receiver. The traditional approach for reducing the magnitude of intermodulation products consists in providing, upstream from the common manifold, a lowpass filter for each channel, so as to eliminate the harmonics of the payload signal; in particular, it has been found necessary to eliminate interference signals at least up to the third harmonic.
In order to reduce the weight and size of the multiplexer, it would be preferable to use a common lowpass filter instead of individual filters for each channel. However, filters known in the prior art do not enable satisfactory filtering to be obtained while simultaneously conveying high power. Waveguide filters adapted for these applications, such as filters of the waffle iron type or corrugated waveguide type present interference passbands above the nominal cutoff frequency, and in particular at frequencies that are harmonics thereof. The magnitudes of these interfering passbands increase with increasing spacing or gap between the walls of the waveguide in the electric field direction of the waves being conveyed, which leads to operation of multimode type: consequently, in order to be effective in eliminating the undesirable frequencies, it is necessary to use filters with a small gap, but that is not possible in high power applications (power of kilowatt or greater order), in particular when the filter is to be used in a vacuum, because of the risk of electron avalanche discharges (“multipaction”). A discussion of the electron avalanche discharge phenomenon can be found in the article by M. Ludovico, G. Zarba, L. Accatino, and D. Raboso “Multipaction analysis and power handling evaluation in waveguide components for satellite antenna applications”, exp, Vol. 1, No. 1, December 2001.